1. Field of the Invention
The present invention relates to a process of producing a multiple-layer glass-ceramic circuit board, more particularly to an improvement in the forming of via-contacts of a multiple-layer glass-ceramic circuit board.
To quickly process a large amount of information, the information processing machines are progressively made smaller in size and larger in processing capacity and LSIs and VLSIS, which are high integrated by miniaturization of electronic elements, are provided for practical use as semiconductor devices occupying the major part of an information processing machine.
These integrated circuits are packaged in such a manner that a plurality of chips of integrated circuits are mounted on a chip-mounting board (interposer) made of ceramic to form an LSI module to be used as a replacement unit mounted on a printed circuit board. Particularly, all of the flip-flop type semiconductor integrated circuits are mounted on a ceramic circuit board.
The ceramic circuit boards on which semiconductor integrated circuits are mounted, are generally used in laminated form, and currently, multiple-layer circuit boards having 20 to 60 laminated layers are provided for practical use. These multiple-layer circuit structures have a via-contact formed of an electroconductive material extending through layers and providing electrical connection between electronic circuits formed in different layers.
Recently, it has been increasingly required that electric signals be transmitted more rapidly between semiconductor devices, and therefore, the circuit conductor be made of a material having a lower electric resistivity and that via-contacts formed in a circuit board be finer to increase the element density of semiconductor devices.
These requirements have led to the development of a glass-ceramic circuit board. To form a circuit conductor of a low resistivity material such as gold, silver, copper or the like, the shrinking temperature during firing of the glass-ceramic board is adjusted to be from 700.degree. to 1000.degree. C. by blending a ceramic with a glass having a softening temperature of 600.degree. to 900.degree. C. To provide high density packaging, the circuit boards have fine via-contacts with a diameter of about 100 .mu.m.
2. Description of the Related Art
A multiple-layer glass-ceramic circuit board is conventionally produced by making a green sheet from a glass-ceramic and an organic binder, forming throughholes in the green sheet at positions at which via-contacts are to be formed, and filling the throughholes with a conductor paste to form via-contacts. A plurality of green sheets composing the respective layers of a multiple-layer circuit board are prepared in the same way.
Each of the thus-prepared green sheets is screen-printed with a conductor paste on the portions including those of the via-contacts or filled throughholes to form an electric circuit pattern, and is then dried.
The dried green sheets are aligned, laminated, and pressed to form an integral laminate body, which is then heated to remove a binder therefrom and fired to provide a multiple-layer glass-ceramic circuit board. The heating for the removal of a binder is carried out at a temperature lower than that at which the subsequent firing is carried out, and is herein referred to as "preliminary firing".
FIG. 1 shows a conventional arrangement for forming via-contacts by using a green sheet 3 having a large number of throughholes or perforations 2 formed therethrough and through a polyethylene terephthalate sheet 1 (commonly called "Mylar sheet") covering the green sheet 3. A suction paper 4 is placed on a filling table 5 provided with an evacuating system (not shown) and the green sheet 3 together with the Mylar sheet 1 placed on the upper surface thereof is overlaid on the suction paper 4.
A conductor paste 15 is placed on the Mylar sheet 1, the evacuating system is actuated, and a squeegee is passed over the Mylar sheet 1 to sweep the conductor paste 15, so that the conductor paste 15 is sucked through the perforations of the Mylar sheet 1 to fill the throughholes 2 of the green sheet 3.
After removing the Mylar sheet 1, the green sheet 3 is screen-printed with a circuit conductor pattern and then dried.
Green sheets 3 corresponding to the respective layers of a multiple-layer circuit board are prepared in the same way, aligned, laminated, and pressed to form an integral laminate body, which is then heated at a relatively lower temperature to effect a preliminary firing and remove a binder from the green sheets 3 and from the conductor paste 15.
The laminate body is then fired at a higher temperature to sinter a glass-ceramic and a conductor metal and form a multiple-layer glass-ceramic circuit board having via-contacts in the laminated layers at the desired positions.
The above-mentioned conventional process uses a conductor paste of a mixture of a conductor metal powder and an organic binder.
The conventional process, however, has the following problems:
(1) Via-contacts frequently contain pores and cracks because of the difference in the shrinkage factors of the conductor metal and the substrate ceramic during sintering thereof because of different sintering behaviors thereof.
(2) Throughholes are difficult to completely fill with the conductor paste because the solvent or binder component of a conductor paste is absorbed by the throughhole wall of a porous green sheet.
(3) When an easily oxidizable metal such as copper is used as the conductor metal, an oxidizing atmosphere cannot be used in the heating for removing a binder, and a reducing atmosphere, such as a humidified atmosphere, must be used instead, under which the removal of a binder is not successful. Accordingly a binder that is not easily thermally decomposed cannot be used in either a conductor paste or a green sheet.
Measures were proposed to solve these problems.
To eliminate the difference between or equalize the shrinking behaviors of the conductor metal and the substrate ceramic, Japanese Unexamined Patent Publication (Kokai) Nos. 61-89839, 62-133002, 63-260199, 63-271995 and 1-201996 proposed the addition of an organic metal to a conductor paste. It is commonly known that the metal powder of a conductor paste is not sintered until the organic binder of the paste is decomposed and dissipated, and is sintered later than the metal powder alone, i.e., the occurrence of sintering and accompanying shrinkage is shifted to a higher temperature region. Moreover, the sintering of the conductor metal occurs abruptly when the organic binder is decomposed and dissipated at a temperature of from 700.degree. to 900.degree. C., and this sintering behavior is quite different from that of the substrate ceramic. The above-recited publications delay the sintering shrinkage of the conductor metal, i.e., shift the occurrence thereof to a higher temperature so that it occurs at the same time as that of the substrate ceramic because the added organic metal is decomposed to form a metal oxide during the preliminary low temperature firing (or the removal of binder) and the subsequent high temperature firing, and the thus-formed metal oxide impedes the sintering of the conductor metal powder in a high temperature region of from 700.degree. to 900.degree. C. Thus, the shrinkage of a conductor paste occurring in a high temperature region of from 700.degree. to 900.degree. C. is controlled by the addition of an organic metal thereto.
Japanese Unexamined Patent Publication (Kokai) No. 61-89839 and Japanese Patent Application No. 2-9018 (by the same assignee as the present application) proposed a process utilizing the same principle, in which a metal oxide, instead of an organic metal, is directly added to a conductor paste.
Japanese Unexamined Patent Publication (Kokai) Nos. 2-18991, 1-281795 and 61-101096 proposed another process in which a conductor metal powder directly fills the throughholes instead of a conductor paste, to avoid an organic substance remaining in the via-contacts and also to improve the packing of the throughholes. Because these processes do not use an organic binder, the dissipation of an organic binder does not occur in the drying and firing steps, the formed via-contacts do not contain pores, and via-contacts of a sufficient packing density can be relatively easily formed in a porous green sheet.
In spite of these improvements, recent refinements in the circuit pattern requires a minute via-contact to have a diameter of about 100 .mu.m and raised the following new problems.
When a conductor paste is used, a complete packing of a 100 .mu.m throughhole is so difficult that neither the addition of an organic metal nor the direct addition of a metal oxide improves the packing density in the throughholes and the pores that occurred during the filling of throughholes remain substantially unchanged in the fired via-contacts. Moreover, because generally having a high thermal decomposition (thermal deterioration) temperature, organic metals often remain in the form of an incomplete chemical compound after the binder removal step or preliminary firing. Particularly, when a conductor metal is composed of an easily oxidizable metal such as copper, a heating must be carried out in a reducing atmosphere unfavorable for removing a binder and the binder remains even after the final firing is completed.
When a conductor metal powder directly fills the throughholes, a refined powder particle is advantageously used to provide a complete packing of minute throughholes. The use of a metal powder alone, however, has a problem in that shrinkage occurs at a temperature lower than that of a substrate ceramic and still lower than that of a conductor paste during the binder removal and the firing, thereby frequently causing a gap between the thus-formed via-contacts and the ceramic substrate because of the different shrinkage factors thereof. Particularly, when the conductor is composed of copper, the heating for removing a binder is carried out in a humidified nitrogen gas atmosphere to prevent oxidization of the copper, and under this atmosphere, the shrinkage of a copper powder begins at a temperature of from 300.degree. to 400.degree. C., which is still lower than that which occurs in a dry nitrogen gas atmosphere. This causes the formation of a gap between the via-contact and the substrate ceramic in a low temperature region of from 400.degree. to 500.degree. C. and the thus-formed gap is larger than that formed when via-contacts are formed with a conductor paste.
To solve this problem, the present inventors have proposed a process in Japanese Patent Application No. 2-9018, in which a conductor metal powder blended with a ceramic powder fills the throughholes. This process effectively prevents the formation of pores at the via-contact/substrate ceramic interface by suppressing the occurrence of shrinkage in a low temperature region of 700.degree. C, or lower, particularly 400.degree. to 700.degree. C., which disadvantageously occurs when a copper powder alone fills the throughholes. This process, however, has a disadvantage in that via-contacts are embrittled and weakened because of the unstable bond between the fired metal particles when a ceramic powder is added in an excessive amount or a ceramic powder has an excessively large particle size.
To solve the problem that a good adhesion cannot be obtained when a conductor particle directly fills the throughholes because of poor wettability thereof with a ceramic, the above-recited Japanese Unexamined Patent Publication (Kokai) No. 2-18991 proposed the addition of chromium oxide to the metal powder. This process, however, has a problem in that the corrosion of the via-contact easily occurs during the firing step and the toxicity of chromium oxide results in low operation efficiency.
It is also noted that the above-recited Japanese Unexamined Patent Publication (Kokai) No. 2-18991 describes the particle size of the copper powder and the chromium oxide (Cr.sub.2 O.sub.3) powder as one tenth (1/10) the throughhole diameter or less, particularly from about one tenth (1/10) to about five-hundredth (1/500).